1. Field of the Invention
This invention relates to a process for the manufacture of hydrofluorocarbons (HFC's) by the reaction of hydrocarbons or fluoro-substituted hydrocarbons with elemental fluorine in the vapor phase.
2. Description of the Prior Art
Chlorofluorocarbons (CFC's) such as CF.sub.2 Cl.sub.2 (CFC-12) are used in industry as refrigerant fluids in heat pumps and compressors. There exists a need to replace CFC-12 and related materials with a compound that does not have a prolonged residence time in the atmosphere. Hydrofluorocarbons (HFC's) are compounds containing only carbon, hydrogen, and fluorine atoms. Certain members of this class of compounds, such as CH.sub.2 FCF.sub.3 (HFC-134a) and C.sub.2 HF.sub.5 (HFC-125), possess the physical properties that allow them to replace products currently in use which are suspected of depleting stratospheric ozone. For example, HFC-134a is now known to be a suitable replacement for CFC-12, and HFC-125 can be used as a propellant and working fluid. Since many of these compounds are not items of commerce, and cannot be synthesized by procedures known in the art, there is a need for new procedures to manufacture these compounds on a commercial scale.
As disclosed in "Organic Fluorine Chemistry" by W. A. Sheppard and C. M. Sharts (W. A. Benjamin, Inc., New York, 1969), fluorinated compounds are typically prepared by the reaction of chloroalkanes or chloroolefins with HF in the liquid or vapor phase usually in the presence of catalysts such as halogen compounds of antimony. It has long been recognized in the art (see, for example, A. L. Henne, Organic Reactions, Vol. 2, p. 56 (1944)) that conversion of --CHCl.sub.2 and --CH.sub.2 C1 groups to --CHF.sub.2 and --CH.sub.2 F groups is very difficult to achieve using these standard reaction procedures.
For example, Henne, et al. (J. Am. Chem. Soc., Vol. 58, pp. 404-406 (1936)) report the conversion of CH.sub.2 ClCCl.sub.3 to CH.sub.2 ClCC1F.sub.2 (HCFC-132b) using an antimony catalyst. In another case, McBee (Industrial and Engineering Chemistry, Vol. 39, pp. 409-412 (1947)) described the reaction of trichloroethylene with HF in the presence of an antimony catalyst to give CF.sub.3 CH.sub.2 Cl (HCFC-133a) and HCFC-132b. The conversion of trichloroethylene to HCFC-133a is also described in British Patent No. 1,585,938 where it is stated that no tetrafluoro compounds were observed. U.S. Pat. No. 4,311,863 pointed out the difficulty of converting HCFC-133a to CF.sub.3 CH.sub.2 F (HFC-134a) which requires high temperatures and special alkali metal fluoride catalysts. U.S. Pat. No. 4,851,595 teaches that HCFC-133a may also be converted to HCFC-134a via SbF.sub.5, a very powerful fluorinating agent; however, conversions were low and contact times were high. In European Patent Application No. 187,643 a tin-based catalysts system is described wherein trichloroethylene is converted to a mixture of HCFC-132b and CH.sub.2 ClCCl.sub.2 F (HCFC-131a). Similar results were disclosed in U.S. Pat. No. 4,258,225 using a TaF.sub.5 catalyst. This reference also taught that perchloroethylene and HF react in the presence of TaF.sub.5 to form a mixture of CHCl.sub.2 CCl.sub.2 F (HCFC-121) and CHCl.sub.2 CClF.sub.2 (HCFC-122); in this case the --CHCl.sub.2 group is resistant to substitution by fluoride.
As can be seen from this prior art, the well-known process of exchange of chlorine for fluorine severely limits the scope of compounds that can easily be prepared; --CH.sub.2 C1 and --CHCl.sub.2 groups are predominant hydrogen-containing functionalities in compounds that can be attained by this technique.
An alternative approach to HCFC's is the chlorination of fluorocarbons such as CH.sub.3 CF.sub.3 (HFC-143a). For example, McBee (Industrial and Engineering Chemistry, Vol. 39, pp. 409-412 (1947)) reported that vapor phase chlorination of HFC-143a afforded a mixture of HCFC-133a, HCFC-132b, CHCl.sub.2 CF.sub.3 (HCFC-123), and the fully chlorinated CF.sub.3 CCl.sub.3 (CFC-113a). McBee also reported the chlorination of CH.sub.3 CClF.sub.2 (HCFC-142b) to give HCFC-132b, HCFC-122, and the fully chlorinated CClF.sub.2 CCl.sub.3 (CFC-112a). Again, using this technique --CH.sub.2 Cl and --CHCl.sub.2 groups dominate the attainable substitution pattern.
HFC's have also been prepared by direct fluorination processes using elemental fluorine. Early work in this field by Calfee, et al. (J. Am. Chem. Vol. 62, pp. 267-269 (1939)) demonstrated that ethyl chloride could be fluorinated in the gas phase to a mixture of CF.sub.4, CF.sub.3 Cl, CF.sub.3 CF.sub.2 Cl, CF.sub.2 .dbd.CCl.sub.2, and CHF.sub.2 CH.sub.2 Cl. Miller (J. Amer. Soc., Vol. 62, pp. 341-344 (1940)) and Bockemueller (Ann. Vol. 506, p. 20 (1933)) reported the fluorination of highly chlorinated alkanes and olefins in the liquid phase to give perhalogenated compounds as well as some higher chloroalkanes.
Fluorinated hydrocarbons have also been made electrochemically. For example, Fox et al. (J. Electrochem. Soc. Electrochem. Technology, Vol. 118, pp. 1246-1249, 1971) reported the electrochemical fluorination of ethane and found that mixtures of fluorinated ethanes were produced. The tetrafluoroethane fraction always contained both CHF.sub.2 CHF.sub.2 (HFC-134) and HFC-134a with the former predominating. Similarly, mixtures of HFC's and HCFC's were obtained by Nagase, et al. (Bulletin of the Chemical Soc. of Japan, Vol. 40, pp. 2358-2362, 1967) when ethane or ethylene were fed into an electrochemical cell with a source of chlorine such as Cl.sub.2, phosgene, or chlorocarbons. Again the tetrafluoroethane fraction contained relatively equal amounts of HFC-134 and HFC-134a.
In a series of papers, Calfee, et al. (J. Am. Chem. Soc., Vol. 59, pp. 2072-2073, 1937; and Vol. 61, pp. 3552-3554, 1939), Young, et al. (J. Am. Chem. Soc., Vol. 62, pp. 1171-1173, 1940), and Maxwell, et al. (J. Am. Chem. Soc., Vol. 82, pp. 5827-5830, 1960) the fluorination of ethane is reported in a variety of fluorination reactors. Although mixtures of fluorinated ethanes were obtained including HFC-134, HFC-134a was not mentioned as a product from these reactions. Cadman, et al. (Transactions of the Faraday Soc., Vol. 72, pp. 1428-1440, 1976) have studied the fluorination of ethanes and HFC's but this work was carried out at extremely low conversion.
Elemental fluorine has also been used to make perfluorinated compounds. For example, in U.S. Pat. No. 4,158,023 a two-step process is claimed for making perfluoropropane, C.sub.3 F.sub.8, by treating CF.sub.2 .dbd.CFCF.sub.3 with HF followed by perfluorination to C.sub.3 HF.sub.7. European Patent Applications 31,519 and 32,210 and U.S. Pat. No. 4,377,715 also disclose and claim processes for perfluorinated compounds. The goal of these investigations has been the preparation of perfluorinated compounds for which elemental fluorine is considered to be well-suited.
It is well known in the art that fluorinations of aliphatic compounds with elemental fluorine can be accelerated by the use of ultraviolet light (e.g., see J. M. Tedder, Chemistry and Industry, Apr., 30, 1955, pp. 508-509). Very little use appears to have been made of chemical initiators for direct fluorinations. For many fluorinations this is not necessary due to the high degree of reactivity of fluorine with many substrates; however, direct fluorination of partially fluorinated materials often requires higher temperatures for acceptable conversions. Miller, et al. (J. Am. Chem. Soc., Vol. 79, pp. 3084-3089, 1957) reported the preparation of CCl.sub.3 CClFCClF.sub.2 from CHCl.sub.3 and CClF.dbd.CClF by co-feeding fluorine; however, none of the chloroform fluorination product, CFCl.sub.3, was apparently produced.
Another means of preparing HFC's known in the art is the reaction of saturated or unsaturated hydrocarbons and fluorinated hydrocarbons with metal fluoride compounds. For example, Burdon, et al. (Tetrahedron, Vol. 32, pp. 1041-1043, 1976) has reported that ethane or ethylene react with cobalt trifluoride to form a mixture of fluorinated ethanes. Again HFC-134a was a minor product along with somewhat greater amounts of HFC-134. Holub, et al. (J. Am. Chem. Soc., Vol. 72, pp. 4879-4884, 1950) reported that CHF.sub.2 CH.sub.2 F (HFC-143) could be fluorinated to a mixture of HFC-134 and HFC-125. Shieh, et al. (J. Organic Chem., Vol. 35, pp. 4020-4024, 1970) have reported that ethylene can be converted to a mixture of fluorinated ethanes using XeF.sub.4 but tetrafluoroethanes were not produced.
Rausch, et al. (J. Organic Chem., Vol. 28, 494-497, 1963) has used cobalt trifluoride and related metal fluorides to convert vinylidene fluoride to HFC-134a. In U.S. Pat. No. 3,346,652 boron trifluoride and trifluoroamine oxide were used to prepare HFC-134a from vinylidene fluoride. Both of these techniques required exotic reagents to prepare HFC-134a.